Micro-morphology and Toxicological Effects of Lunar Dust

Introduction: The Moon is being considered as an effective test-bed and fueling station for future space missions to Mars and beyond, with In-Situ Resource Utilization (ISRU) of lunar materials [1]. From the Apollo missions, however, it is well known that dust on the Moon can cause serious problems for human’s exploration activities [2]. One of the paramount problems is the potential for a toxic nature of lunar dust, the <20 μm fraction of the regolith. In 1972, two astronauts Harrison Schmitt and Gene Cernan, commander of Apollo 17, reported that the air in their Lunar Module (LM), the Challenger, smelled like “gun-powder”. The two astronauts had just returned from their first EVA (Extra-Vehicular Activity) with dusty spacesuits. In the LM, the dust became airborne and was breathed by them. Later, Schmidt complained of congestion and fever, although his symptoms went away shortly. Armed with our knowledge of the chemical and physical properties of the lunar dust [3], it has become apparent that there may be acute toxicological effects to humans with breathing of the lunar dust. An important first step in dealing with dust toxicity is to fully characterize the morphology, size distribution, and reactivation surface area of dust particles. We shall briefly describe about the possible toxicity and particle size and shape of lunar dust, and explain the detailed experimental methods of particle-size measurement, and report on the particle-size distribution (PSD) and reactivation surface areas. Toxicity: With terrestrial interests in dust toxicity, there have been numerous studies in PSDs and health effects of dust particulates in air pollution [4,5]. In fact, dust particles on Earth have been shown to result in pulmonary diseases such as silicosis, asbestosis, and black lung disease with miners, in general. In the case of ultra-fine (<0.1μm) particulates, they could be easily inhaled and deposited in alveolar sacs and ducts of human lungs, and might be a cause of a progressive lung failure or cancer by a fibrogenic reaction, and pass into the blood stream, or enter into the lymphatic system [2]. Particle Size (Granulometry): The sizes of inhalable particles are distinguished with different ranges of particle diameters: coarse (D >2.5μm), fine (2.5μm > D > 0.1μm), and ultra-fine (D <0.1μm) [6]. The granulometry of particles can be determined by different methods such as sieve analysis, image analysis, laser analysis, etc. In this study, the microscopic imaging method using scanning electron microscopy (SEM) was used to measure the PSD of lunar dust particles in Apollo 17 soil, 70051. Particle Shape (Morphology): Figure 1 shows SEM images of lunar dust particles in Apollo 17 soil, 70051. Most of particles show sub-angular to angular shapes with sharp edges. There are four prominent shapes: 1) spherical; 2) angular blocks; 3) glass shards; and 4) irregular (ropey or Swiss-cheese). In particular, submicron bubbles and cracks are present in most grains. This causes a multiplication of the reactivation surface area. In case of lunar soil simulant (JSC-1), the similar morphology of grains was observed [7], minus the submicron bubbles and the swiss-cheese morphology. It is obvious that considerations for lunar dust mitigation must factor in the sharp edges of the glass fragments and unique morphological characteristics of lunar dust particles [8].